2592
W . C. VOSBURGH,P. F. DERR,G. R. COOPER AND R. G.
Laue data, and, considering both, the value of the with a F-H-F distance may be given as 2.26 probable error of not more than 0.01 A.
a.
Discussion In going from the HF2- ion in crystalline potassium bifluoride to the (HI?)* polymers in gaseous hydrogen fluoride there is a lengthening of 0.29 A 0.06 A. in the F-H-F distance, This shows that formation of additional hydrogen bonds lengthens the F-H-F distance. While the value 2.37 * 0.1 A.for the F-H-F distance in ammonium bifluoride leaves some doubt as to a lengthening, it is strongly indicated that the formation of two additional N-€3 . . . F bonds causes a weak-
[CONTRIBWION FROM THE DEPART-NT
BATES
Vol. 61
ening of the F-H-F bond and a probable lengthening of about 0.1 A. The theoretical value calculated by PaulingKfor this distance is 2.32 8. with a symmetrical linear configuration for FH-F.
Summary A redetermination of the parameter in potassium bifluoride has been carried out using Laue and oscillation photographs. The value u = 0.1408 j= 0.003 leads to the fluorinduorine A comdistance 2.26 * 0.01 A. in IF-H-F]-. parison is made with other reported values. ( 5 ) L. Pauling, "The Nature of the Chemical Bond," Cornell University Press, Ithaca, N. 1 ' , 1939, p 277.
PASADENA, CALIFORNIA
RECEIVED JUNE 26, 1939
OF CHgbUSTRY OF DUKEUNYERSITY AND THE STERLING CHEMISTRY TORY OF YALE UNIVERSITY]
LABORA-
Silver and Mercurous iodide Electrodes BY WARREN C, VOSBURGH,PAULF. DERR,GERALD R. COOPER AND ROGERG. BATES'
The silver and mercurous iodide electrodes have both been shown to be reproducible under favorable conditions,2 and it seemed desirable to atn of the two. This was t, and the result can be considered only approximate. Some of the observations made in the course of the study are of value, however. SilverIodide Mectrode.-AJl of the silver iodide electrodes were prepared by the thermal method; some from silver oxide and iodate2"and some from oxide and iodide.2b In the preparation of some, the platinum wire was electroplated with silver before application of the paste, but this was found unnecessary and was omitted in the later electrodes. Silver iodide electrodes agreed within 0.02 mv. when compared in same vessel with oxygenfree potassium iod solution (about 0.02 m ) as the electrolyte. The agreement was better on the second day than on the tirst, and continued good indefinitely. The agreement was not as good when the electrolyte was saturated with lead iodide or when 0.01 m cadmium iodide was lyte. When air was admitted to a cell consisting of two silver iodide electrodes (1) Sterling Fellow at Yale University, 1937-39. S 6'7, 1528 (1935); (b) Bates, ibid., (2) (a) Owen, T H ~JOURNAL, 60, 2984 (1938); (e) Vosburgh, ibid., 60, 2391 (1928); (d) Bates and Vosburgh, a b i d , bB, 19x4 (19371
in the same vessel, it caused disagreement of the electrodes.8 A trace of mercurous iodide also caused disagreement. Mercurous Iodide Electrodes.-While the mercurous iodide electrode is highly reproducible when the electrolyte is a saturated cadmium iodide it was found much less reproducible when the electrolyte was a potassium iodide solution. Two mercurous iodide electrode systems in a single H-vessel prepared with nitrogen-vacuum technique gave a cell with an electromotive force anywhere from zero to well over a millivolt in spite of considerable care to avoid errors. When one electrode had a thick (1 to 2 cm.) layer of mercurous iodide paste covering the mercury and the other a thin (0.5 cm.) layer, the thick-layer electrode was always positive, and the electromotive force varied from 0.3 mv. to nearly 2 mv. in Werent cells and a t different times. When both layers of mercurous iodide were thin, the electromotive force varied from zero to 0.3 mv. When the electrolyte was a 0.01 or 0.02 rn cadmium iodide solution, the effect of thick and thin layers was not as pronounced. Solid lead iodide at both electrodes, with a potassium iodide electrolyte, reduced the effect of thick and thin layers, also. (3) (a) Smith and Taylor, R ~ i m ' R iCkcm., 18, 762 (1988): (b) Taylor and Smith, J . Resaarch Not!. Bur. Standards, 44, 307 (1939).
Oct., 1939
SILVER AND
MERCUROUS IODIDE ELECTRODES
The effect of thick and thin layers may possibly be explained on the basis of the reaction
2593
electromotive forces of these on the first day varied between 0.1119 and 0.1141 v. at 30°, the silver iodide electrode being negative. HgzIz 4-21- J _ H g 4-HgL' The highest cells decreased during the next few This reaction takes place when mercurous iodide days and most of the lower ones increased. For and sufficient iodide ion come together, and there18 cells that varied less than 0.2 mv. during the fore the iodide ion concentration a t a mercurous lirst four days the range was 0.1127 to 0,1136 v. iodide electrode is not the same as in the main This is within the range of variability of merbody of the electrolyte. If the reaction is slow, curous iodide electrodes with different thicknesses diffusion would be able to keep the iodide ion of the mercurous iodide layer. Three of the cells concentration a t the thin-layer electrode larger were measured over a period of twenty-four to than a t the thick-layer electrode. A larger forty days. One of these started a t 0.1141 v. iodide ion concentration means a smaller merand decreased to 0.1131 v. On shaking the mercurous ion concentration and consequently a more curous iodide electrode, the value dropped to negative electrode. Mercurous iodide electrodes 0.1128 v. and remained constant thereafter for are reproducible in saturated cadmium iodide twenty-five days. Another cell started a t 0.1130 solutions because the small iodide ion concenv., decreased in four days to 0.1128 v. and retration* is held practically constant by the varimained constant for thirty-six days. Its elecous equilibria in the solution. If this theory tromotive force was not affected by shaking. The is correct, the value assigned to the normal pothird was like the first, except that its initial electential of the mercurous iodide electrode4 must be tromotive force was not as high. All three cells considered less certain than the precision of the gave the value 0.1128 v. after shaking. measurements would indicate. Cells.-A large number of cells were made, each with one silver iodide and one mercurous iodide electrode. When made in ordinary Hshaped vessels the constancy was not satisfactory. Diffusion of the mercurous iodide to the silver iodide electrode was probably responsible. A plug of glass wool to retard diffusion helped, but did not overcome the trouble, and the special cell vessel illustrated in Fig. 1 was designed. The electrode systems were placed in parts A and B, with the use of nitrogen-vacuum technique. The Fig. 1. tube C was not filled with electrolyte until a measurement was to be made. It was then filled If the differences between cells is ascribed in through tube D with air-free solution from a part to differences in thickness (or denseness of flask like that described by Brown and MacInnes.S packing) of the mercurous iodide layer, the lower After a measurement, the solution in C was exelectromotive forces should correspond to cells in pelled through F by nitrogen pressure. Some of which the layer is thin. The effect of shaking is the vessels used had interchangeable groundprobably to aid diffusion, and this lowers the glass joints at C. This allowed an interchange electromotive force to the thin-layer value of of electrodes between cells. A few vessels had 0.1128 v. two legs a t A and two a t B, allowing the preparaA group of cells made with 0.01 m potassium tion of duplicate electrodes of each kind. Cells iodide solution was less satisfactory, but the best made in the vessels having the joints a t C were ten varied between 0.1124 and 0.1130 v. between maintained at constant temperature in an air-bath the second and fourth days, with an average of and the others in,an oil-bath. 0.1127 v. A number of cells made with an elecOf a large number of cells made with 0.02 m trolyte of 0.02 m potassium iodide solution satupotassium iodide solution as the electrolyte, 48 rated with lead 'iodide were variable and unsatiswere selected as the most satisfactory. The factory, except for one cell that was constant at (4) Bates and Vosburgh, THISJOURNAL, 69, 1189 (1937). 0.1128 v. The use of a dilute cadmium iodide ( 6 ) Brown and MacInnes, ibid., 6T, 1358 (1935).
HARRYSELTZ AND BERNARD J. DEWITT
2594
solution as the electrolyte gave cells that were rather variable and lower than 0.1128 v. Saturated cadmium iodide gave cells that started very high and decreased continuously. The bad effect of cadmium iodide is undoubtedly on the silver iodide electrode. In connection with a study of the thermodynamics of zinc iodide in aqueous solution by means of zincsilver iodide cells,2bsome cells with mercurous iodide electrodes were made from which the electromotive force of the silver-mercurous iodide cell could be calculated. The calculated values are given in Table I. The results are in TABLE I CALCULATED ELECTROMOTIVE FORCEOF THE CELL Ag/AgI, ZnIz(m)lZnIe(m), HgzIdHg ZnIn, m
Ets
Eno
Esa
Em
Vol. 61
Bates and Vosburgh* found for the normal potential of the mercurous iodide electrode the value 0.0405 v. a t 25'. They used rather thin layers of mercurous iodide and a molality of potassiuni iodide of about 0.01 and their value can be combined with 0.1112 v. to give the approximate value 0.1517 v. a t 25" for the normal potential of the silver iodide electrode. Owen2" found 0.1522 v. and C a m and Taylor6 found 0.1510 v. In view of the uncertainty of the value 0.1517 v., it cannot be considered to be in serious disagreement with either of these. Some recent measurements by Gould and Vosburgh of the silver iodide electrode against a hydrogen electrode are in as good agreement with 0.1517 v. as could be expected. Summary
E.
The cell Ag/AgI, I-/I-, Hg&/Hg was found to be not as reproducible or constant as experience with the two electrode systems in other cells would lead one to expect, but the most probable value based on a large number of cells leads to an approxapproximate agreement with those described imate value for the silver iodide electrode. above. They show that the electromotive force The trouble with the silver-mercurous iodide depends on the electrolyte concentration, which cell may be summarized by saying that while each is in agreement with the theory proposed above of the electrode systems is highly reproducible for the variability of the mercurous iodide elec- under favorable conditions, no single electrolyte trode. Table I and also some measurements of was found that did not affect one or the other of the previously described cells show that the tem- the electrodes unfavorably. perature coefficient is about 0.00030 v. per degree; (6) Cann and Taylor, THIS JOURNAL, 59, 1484 (1937). 0.1127 v. a t 30' corresponds to 0.1112 v. a t 25'. DURHAM, l\i. c. RECEIVED JULY 19, 1939 0 01075 0.1076 0.1091 0.1106 0.1122 0.1139 ,01474 .lo81 .I096 ,1112 ,1127 .1144 01801 1086 ,1102 .1117 .1134 .1150 03373 . . 1125 ,1141 . 06068 1117 .1132 1147 ,1163 ,1179 ~
- .__ __ [CONTRIBUTION FROM THE
_ I
DEPARTMENT OF CHEMISTRY,
CARNEGIE INSTITUTE OF TECHNOLOGY]
A Thermodynamic Study of the Lead-Antimony System BY HARRYSELTZ AND BERNARD J. DEWITT In the hope that the accumulation,of data on the thermodynamic properties of binary liquid metal solutions may lead eventually to a better understanbing of the nature of such systems, investigations along these lines are being continued in this Laboratory. In this paper the results of a study of the lead-antimony system are described. The electromotive force values of cells of the type Pb(l)/PbC12 in KCl-LiCi(l)/Pb-Sb(l)
were measured, u the purified metals and technique described in previous publications. (1) Strickler and Seltz, THISJOURNAL, 18,2084 (1936). (2) Seltz and DeWitt, ibid., 60, 1305 (1938).
In this work it was found possible to extend the e. m. f. readings in rather thick-walled Pyrex Hcells under vacuum up to 630" without collapse of the cells. This permitted measurements to a mole fraction of lead of 0.1 and eliminated any difEculty in the extrapolation of the activity and partial molal relative heat content curves.
Experimental Data In Table I, the experimental results are tabulated, along with the calculated activities and relative heat contents for lead at the various mole fractions. (The subscript 1 will be used to designate values for the lead comDonent.1 At the
